This was a multicenter study designed to determine whether the vertical transmission of HIV can be interrupted by continuous oral zidovudine (ZDV) administration to HIV infected pregnant women beginning at 14 to 34 weeks of gestation, an intravenous intrapartum infusion, and oral ZDV administration to their infants from birth to six weeks of age. This study also evaluated the safety of zidovudine for both the pregnant woman and the newborn infant. From April 1991 through December 20, 1993, the cutoff date for the first interim analysis of efficacy, 477 pregnant women were enrolled. During the study period, 409 gave birth to 415 live-born infants. HIV infection status was known for 363 births (180 in the ZDV group and 183 in the placebo group). Thirteen infants in the ZDV group and 40 in the placebo group were HIV infected. The proportions infected at 18 months were 8.3% in the ZDV group and 25.5% in the placebo group. In conclusion, in pregnant women with mildly symptomatic HIV disease and no prior treatment with antiretroviral drugs during the pregnancy, a regimen consisting of ZDV given ante-partum and intra partum to the mother and to the newborn for six weeks can reduce the risk of maternal-infant transmission by approximately two thirds. Based on the above results, the study was closed to further enrollment February 18, 1994.